Thomas Gray arrives at the café just in time. He’s had to nip into H&M to get a new shirt because he lost his last night at a Mumford & Sons gig after-party. His red jumper is badly stretched because “it had a lot of people in it last night”.

Gray, 25, has been friends with Marcus Mumford since their time at Edinburgh University. He toured with them selling band T-shirts but is becoming a star in his own right, which is good because by his own admission he wasn’t much good with merchandise, accidentally leaving 4,000 T-shirts in Stockholm — “nightmare”.

His tongue-in-cheek video Stag Do has made him a YouTube sensation with nearly half a million views. He’s been meeting agents and is spending most of this month at New York’s Upright Citizens Brigade Theatre, a comedy school that’s a feeder for Saturday Night Live.

Yet Gray has never done a live gig. Instead, he records sketches in his bedroom on his cracked iPhone and uploads them to YouTube.

He believes the internet is under-used for comedy. “It’s such an obvious thing — it’s where everyone watches TV. You don’t flick through your Radio Times any more. Kids come home from school and click on YouTube. I wanted to broadcast myself and thought this was the best way to do it.”

Gray’s videos were online for six months before anyone noticed them. “It was just a couple of my friends commenting how they didn’t like them.” Then he went on a stag do in April and almost instantly got 1,000 views. “It was bizarre. I’ve no idea how it happened.” His videos are funny, in a Steve Coogan-meets-Jack Whitehall type way. Gray says he plays deluded characters who are “a mix of extreme confidence and extreme ignorance about themselves”. It’s the sort of thing people want to show their friends.

Loyally, he won’t say whose stag he was at but Mumford married Carey Mulligan in April and Gray does admit it was a friend his age. Shortly afterwards, YouTube invited him to be a partner, which lets you earn money from your videos and gives you resources to edit and improve them. “I felt so important when I got the email — but it currently earns me about £120.” A month? “In total. It’s not as good as people think.”

Gray has been supplementing his income by teaching history three days a week at a school in Sunderland, where his family live, and coming down to London, couch-surfing in Clapham and Fulham for the rest of the week to work on comedy. But the videos have diminished his authority in class and with pupils’ parents, who spend most of parents’ evenings complimenting him on his comedy work. Yet as a teacher, he knows his stuff and can name every king and queen of England and give the dates they reigned. I bring along a short test and he scores impressively. “Teaching’s fun. I’ll miss it if I have to stop. I like the kids.”

This year Gray wants to quit smoking, perform live, play more football and find a girlfriend. “I’ll like a girl, meet them, secretly be in love with them for about three years, confess it too late, then move on to the next one. I’m not very good at it.”

Lots of his comedy is based on this inability to talk to girls. “My friends are similar,” he says. “We sit there and say , ‘let’s go to a place where there are girls’ and then we end up sitting in the corner not talking to them. Maybe it’s shyness. I don’t know.”

What about Carey Mulligan’s friends? “Lots of them are unattainably fit.”

It’s not just Gray’s circle who do this. He tells a story about meeting Prince Harry at a party. “He walked in wearing a waistcoat with a whistle round his neck and said jokingly, ‘I thought you were bringing the fit girls?’”

But they love Englishmen in America. “I’ll dress smart, act posh, walk into Starbucks and loudly order a pot of tea. That’s what they like, right?”